1. Technical Field
The present invention relates generally to the field of insulation materials and enclosures for thermal isolation. More specifically, but without limitation, the present invention relates to insulation structures with high thermal resistance and mechanical stability.
2. Description of Related Art
Thermal insulation generally refers to materials used to reduce the rate of heat transfer, or the methods and processes used to reduce heat transfer. Many different materials can be used as insulators. Typical properties of insulation materials include high porosity, low conductivity, and low density. Organic insulators may be made from petrochemicals and recycled plastic. Inorganic insulators may be made from recycled materials such as glass and furnace slags. Insulation materials are also produced in various forms and compounds, such as solid, powdered or granular form, fumed silica, layered sheets, aerogels, etc.
Aerogels describe a class of material based upon their structure, namely low density, open cell structures, large surface areas and nanometer scale pore sizes. Aerogels are a solid material derived from gel in which the liquid component of the gel has been replaced with gas. They have nanometer-scale pores. The nano-scale lattice structure and pores create a reduced mean free path for gas molecules thereby reducing energy and mass transport. Aerogels are highly porous, constituting 90% to 99% voidage. Aerogel products are commercially available from companies such as Cabot Corporation™ (information available at www.cabot-corp.com).
The passage of thermal energy through an insulating material occurs via three mechanisms: solid conductivity, gaseous convection, and radiative (infrared) transmission. The sum of these three components gives the total thermal conductivity of the material. Solid conductivity is an intrinsic property of a specific material.
Highly porous bulk or solid materials such as Aerogels are remarkable thermal insulators because they almost nullify the three methods of heat transfer (convection, conduction, and radiation). For dense silica, solid conductivity is relatively high. However, silica aerogels and other highly porous silica based materials possess a very small fraction of solid silica. The solids that are present consist of very small particles linked in a three-dimensional network. Therefore, thermal transport through the solid portion of silica aerogel and these types of materials occurs through a very tortuous path and is not particularly effective. The space not occupied by solids in highly porous materials is normally filled with air (or another gas) unless the material is sealed under vacuum. These gases can also transport thermal energy through the porous material. The other mode of thermal transport through these types of materials involves infrared radiation. Silica aerogels are reasonably transparent in the infrared spectrum. At low temperatures, the radiative component of thermal transport is low, and not a significant problem.
At higher temperatures, however, radiative transport becomes the dominant mode of thermal conduction. Carbon aerogel is a good radiative insulator because carbon absorbs the infrared radiation that transfers heat. Elements with radiation reflective properties (reflectors) are often used to reduce radiative heat transfer. Reflectors may be added or combined with insulating materials to address the issue of radiative heat transport.
Operating thermal insulation systems under vacuum also reduces thermal conductivity. Vacuum insulations are commonplace in various products (such as Thermos bottles). In the case of materials such as aerogels, it is only necessary to reduce the pressure enough to lengthen the mean free path of the gas relative to the mean pore diameter to attain reduced conductivity. Other insulation systems often require a high vacuum to be maintained to achieve the desired performance.
One type of thermal insulating structure known as a Dewar flask has been in use since the late 1800's and has served multiple purposes in many fields. In the field of subsurface and hydrocarbon exploration, Dewar flasks are used differently than the original Dewar. Although they serve the same purpose of insulating the internal contents from external heat, Dewars used in oilfield applications are subjected to much higher temperatures than Dewars used in other areas and they are subject to more restrictive dimensional and physical requirements. Downhole tools are exposed to difficult environmental conditions. The average depth of wells drilled each year becomes deeper and deeper, both on land and offshore. As the wells become deeper, the operating pressures and temperatures become higher. Downhole conditions progressively become more hostile at greater depths. At depths of 5,000 to 10,000 meters, bottom hole temperatures of 260° C. and pressures of 200 Mpa are often encountered. These deep well conditions of high pressure and high temperature damage the external or exposed tool components, and subject the internal electronics to excessive heat.
Conventional Dewar assemblies used in the oilfield industry consist of elongated tubular structures, typically metallic, having a central open bore to house the component(s) to be protected from external heat. U.S. Pat. Nos. 872,795, 1,497,764, 2,643,021, 2,776,776, 3,265,893, 3,481,504, 3,435,629, 4,560,075, 6,336,408, 4,722,026, 4,375,157, 4,440,219, 4,513,352, 2,643,022 and U.S. Patent Application Publication No. 20050208649 describe Dewar-type thermal housings. Dewar flasks used for subsurface or downhole tools are common in wireline applications (See, e.g., U.S. Pat. No. 4,078,174). They are commonly used to protect electronics and other temperature sensitive parts of the equipment from high temperatures encountered within a wellbore.
The use of Dewar flasks for drilling applications is less common because of issues with shock and vibration. During drilling operations, a downhole tool is subjected to high shock and stress on the tool body as the tool is rotated and pressed while traversing through earth formations. Conventional Dewar flasks consist of an outer and inner shell. Contained between the shells is MLI (multi layer insulation) in high vacuum. The multilayer insulation usually consists of a composite of materials including reflectors for radiation such as thin aluminum sheets or metalized films, and fiberglass layers to separate the reflectors. The materials are layered in the annular space with vacuum space in between them. High vacuum in between the layers is used to reduce conduction and convection. U.S. Pat. Nos. 3,038,074, 4,340,305, 3,007,596 describe thermal housings incorporating layered insulation. U.S. Pat. Nos. 6,877,332, 6,672,093, 6,614,718, 6,341,498 and U.S. Patent Application Publication No. 20070095543 describe Dewar assemblies having aerogel composite insulation.
In order to protect the Dewar flasks from shocks and vibration, conventional designs use centralizers to support the inner shell from the outer shell within the assembly. One type of centralizer is sometimes referred to as a wagon wheel. Other centralizer techniques entail the disposal of stainless steel bands on the inner sheet to provide support. There are several problems with using conventional centralizers. One problem is that the centralizer provides a thermal path for the heat to pass from the outer shell to the inner shell, thus reducing the effectiveness of the flask. Another issue with the use of a centralizer is that the inner shell is not fully supported along its length. When the flask is subjected to shock this allows the inner shell to flex, placing additional stress on the assembly and potentially causing fractures leading to the loss of vacuum. Loss of vacuum greatly reduces the effectiveness of the flask. Another common failure caused by shock and vibration is the breakdown of the MLI material. Examination of the insulation structure inside conventional MLI filled Dewar flasks has shown that compressive loading from repeated shocks causes deformation and deterioration of the MLI, resulting in degradation of thermal insulation values, U.S. Pat. Nos. 1,071,817, 2,862,106, 2,396,459 propose using granules and powders as insulating material.
A need remains for improved insulation techniques for use in high temperature and cryogenic environments.